The short answer
NSF/ANSI standards are voluntary national standards for home water treatment. Each covers a type of product or claim: 42 for taste and odor, 53 for health-related contaminants, 44 for softeners, 55 for UV, 58 for reverse osmosis and 401 for emerging contaminants. Certification applies to specific contaminant claims on a specific model, so always check the listing for the contaminant you care about.
Why these standards exist
No federal regulations govern residential water filters, purifiers or reverse osmosis systems. Instead, voluntary national standards set minimum requirements for their safety and performance.[source] Products are tested against those standards by third-party certifiers. NSF also points out that the numbers reflect the order in which standards were written; they aren't a ranking, so 58 isn't "better" than 42.[source]
The six standards you'll see most
| Standard | What it covers | Example claims |
|---|---|---|
| NSF/ANSI 42 | Aesthetic (non-health) effects, for point-of-use and whole-house systems[source] | Chlorine, taste and odor, chloramine, particulate, iron, manganese, zinc, TDS[source] |
| NSF/ANSI 53 | Health-related contaminants[source] | More than 50 possible claims, such as lead, Cryptosporidium, VOCs, chromium[source], and PFOA/PFOS[source] |
| NSF/ANSI 44 | Water softeners using cation exchange resin regenerated with sodium or potassium chloride[source] | Hardness reduction |
| NSF/ANSI 55 | Ultraviolet systems[source] | Class A: inactivate bacteria, viruses and cysts in contaminated water. Class B: reduce non-disease-causing bacteria in already-disinfected water[source] |
| NSF/ANSI 58 | Reverse osmosis systems[source] | Contaminants regulated by EPA and Health Canada[source], including PFOA/PFOS and "Total PFAS" claims[source] |
| NSF/ANSI 401 | Emerging compounds and incidental contaminants[source] | Up to 15 specific claims, such as certain prescription drugs, herbicides and pesticides[source] |
A few related ones you may run into: NSF/ANSI 62 covers distillation, NSF/ANSI 177 covers shower filters (certified only to reduce free available chlorine), and NSF/ANSI 244 covers filters for occasional microbiological contamination of otherwise safe public water.[source]
UV: why Class A vs. Class B matters
If you're buying UV disinfection for a well with a bacteria problem, Class A is the one that matters: it's the class for disinfecting contaminated water. Class B is for reducing nuisance bacteria in water that's already been disinfected.[source] A UV unit without a class stated hasn't told you much.
Certification is claim by claim
This is the part that trips people up. NSF says certification to a standard doesn't mean a product reduces all possible contaminants; you need to confirm it's certified for the contaminants that concern you.[source] Under NSF/ANSI 42 and 53, the specific reduction claims vary product by product.[source]
- A filter certified to NSF/ANSI 53 for lead may have no PFAS claim at all.
- A filter certified to NSF/ANSI 42 for chlorine isn't automatically certified for chloramine; they're separate claims.[source] Our chloramine guide explains why that matters locally.
- An RO system may combine several standards. EPA notes an RO unit with added filters or UV could be certified to 42, 53 or 55 as well as 58.[source]
- Certification applies to the exact model and replacement cartridge. A look-alike cartridge or a different model number isn't covered.
How to look up a product yourself
- Step 1
Get the exact model number
Write down the model of the system and its replacement cartridges or membrane, not just the brand.
- Step 2
Search the NSF listings
Go to NSF's certified drinking water treatment unit listings[source] and search by brand, model or the contaminant you care about.
- Step 3
Or search WQA's Gold Seal listings
The Water Quality Association also certifies products to NSF/ANSI standards through its Gold Seal program.[source] Search its certified product database.
- Step 4
Read the claims, not just the standard
Find your contaminant (lead, arsenic, PFOA/PFOS, chloramine) in the model's list of reduction claims. If it isn't there, the product isn't certified for it.
- Step 5
Plan the upkeep
Certified performance assumes parts are maintained. EPA advises replacing filters on the manufacturer's schedule, and RO membranes may need replacing over time.[source] That's what our Care Plans are for.
Other certifiers: WQA Gold Seal
NSF isn't the only certifier. The Water Quality Association's Gold Seal program tests and certifies drinking water treatment units, softeners and components to NSF/ANSI standards, and WQA holds accreditation for that work from ANAB and the Standards Council of Canada.[source] WQA's Gold Seal program has been certifying products since 1959 and is accredited by ANSI.[source] A product listed by an accredited certifier against the right standard and claim is what you're looking for, regardless of which certifier's mark it carries.
When we recommend equipment, we'll show you the listing for each claim that matters to your water before you buy. We haven't chosen product lines yet, so we make no certification claims about specific products here. To see how we decide what your home needs, start with our process or a water test, and see things we won't sell you.
Sources
- NSF, NSF Standards for Water Treatment Systems (2026)
- NSF, NSF/ANSI 42, 53 and 401: Filtration Systems Standards (2025)
- NSF, Forever Chemicals and the Advancement of Filtration Standards (2024)
- Water Quality Association, Product Certification (Gold Seal program) (2026)
- U.S. EPA WaterSense, WaterSense Guide to Selecting Water Treatment Systems (November 2024) (2024)